It is known that an anionic polymer can be produced by anionically polymerizing monomers such as a conjugated diene, an aromatic vinyl compound, or the like, using an organic alkali metal compound or a living polymer formed by a polymerization of a conjugated diene, an aromatic vinyl compound, or mixture thereof. Further, it is known that the heat resistance, the oxidation resistance, the weather resistance, the ozone resistance, and the like of a polymer can be improved by hydrogenating an unsaturated double bond derived from a conjugated diene using a hydrogenation catalyst such as a nickel-based Ziegler catalyst and a titanium catalyst. These are industrially used as synthetic rubbers (see PTLs 1 to 12).
The temperature for anionic polymerization is usually 20° C. to 110° C., and as the polymerization temperature is higher, the polymerization rate increases. However, in a case where the polymerization temperature is higher than 110° C., the polymerization is terminated in the middle of the process, and as a result, a polymer having a broad molecular weight distribution is produced, and thus, the mechanical properties of the obtained polymer are deteriorated. Further, it is also known that in a case of an anionic polymer using a conjugated diene as a monomer, its mechanical properties are controllable by a binding mode of the conjugated diene, and the binding mode is controllable by the type and the amount of a Lewis base which coexists during the polymerization of the conjugated diene, and also by the polymerization temperature. Therefore, in order to produce an anionic polymer having excellent mechanical properties, it is important to rapidly remove the heat of the polymerization reaction to control the temperature in the anionic polymerization reaction (see PTL 1).
As a complete mixing-type reaction vessel capable of quickly removing the heat of the polymerization reaction, a reaction vessel having a cooling coil provided therein is known. However, due to the complex structure of such a cooling coil and the high viscosity of a reaction liquid containing a polymer, a sparingly soluble polymer, a gel, and the like are deposited in the cooling coil. A molded article using an anionic polymer containing such a sparingly soluble polymer or gel has a remarkably reduced product value in terms of the concave-convex surface and the uneven transparency. Further, a reaction vessel having a type providing a cooling coil therein has a problem in that the ratio of the heat transfer area of the cooling coil to the volume of the reaction liquid decreases due to an increase in the size. For this reason, a method in which a reflux condenser is provided in a complete mixing-type reaction vessel to condense and cool monomers or a solvent is provided (see PTLs 9 to 12 and NPLs 1 and 2).
PTL 9 discloses a method for producing a styrene-butadiene copolymer, in which at the time of polymerizing butadiene and styrene in the presence of an ether compound or a tertiary amine compound using an organic lithium compound in a hydrocarbon solvent, batch polymerization is performed using a reflux-type self-cooling reaction vessel equipped with a condenser, a random styrene-butadiene copolymer which does not substantially include a polystyrene block chain in the terminal of the polymer is produced, and then a coupling reaction is performed such that a copolymer produced using a halogenated tin compound includes at least 10% of a polymer including a carbon-tin bond chain. According to PTL 9, it is described that since the inside of a polymerization system can be cooled by the evaporation reflux of butadiene, it is easy to suppress an increase in the temperature which may be caused by the heat of the polymerization reaction, and the maximum temperature to which the polymerization system reaches can be kept at 120° C. or lower.
PTL 10 discloses a method for producing at least one kind of conjugated diene and at least one kind of vinyl aromatic compound, in which a solvent including at least 60% by weight of cyclopentane is used, and the heat of the polymerization reaction is recovered by the reflux cooling of cyclopentane.
PTLs 11 and 12 disclose a method in which at the time of performing solution polymerization of a block copolymer including an aromatic vinyl compound and a conjugated diene compound using an organic metal as a polymerization initiator, using a reactor having a space occupied by solvent vapor, monomer vapor, and an inert gas present in the upper part of the reactor during the polymerization reaction, a gas in a gas phase portion of the upper part of the reactor is guided to a heat exchanger provided in the outside of the reactor, a liquid mainly composed of a solvent condensed in the heat exchanger is returned to the gas phase portion of the reactor, and a gas mainly composed of inert gases, not condensed, is forcibly returned to a liquid phase portion in the lower part of the reactor to control the polymerization, and also disclose that by such a method, the heat of the polymerization reaction generated is removed mainly by the latent heat of the solvent evaporated.